It is well known in the art to reinforce rubber with metal elements. These reinforced rubber compositions are applied in the preparation of various articles such as hose, belts, tires and rubber vibration insulators. In particular, the incorporation of metal wire cords into pneumatic tires of the radial ply type has found wide application. It is, of course, of the utmost importance that the bond between the rubber and the metal should be as strong as possible and remain intact over a long period of time, even under severe conditions. One of the problems which has been recognized is that corrosion of the metal surface may occur under the influence of moisture. This, of course, adversely affects the rubber-to-metal bond strength. To achieve satisfactory rubber-to-metal bonding various adhesion promoters have been suggested and developed.
For instance, German Patent Application No. p 33.232 suggests the use of zinc salts of unsaturated or saturated fatty acids, resin acids, naphthenic acid, oxidized paraffinic acids or mixtures thereof. Canadian Patent Specification 895371 describes rubber compositions comprising a tackifying amount of a Group I or II metal resinate. German Patent Specification 898 809 teaches the use of cobalt naphthenate as rubber adhesion promoter.
According to U.S. Pat. No. 4,203,874 use may be made of a cobalt rubber adhesion promoter having the formula: EQU (R--CO.sub.2 --Co--O).sub.3 B,
wherein R is an alkyl radical of from 9 to 12 carbon atoms. This promoter is commercially available under the tradename Manobond C (Wyrough and Loser Inc., Trenton, N.J.).